Gas lift system



C. R. LUIGS GAS LIFT SYSTEM May 9, 1967 Filed Oct. 30, 1963 HA J m Lm Hun m -w u m mami lmm N Q H Qw who U M a C HARLES R. Lu/as I NVEN TOR.

ATTORNEYS United States Patent 3,318,258 GAS LIFT SYSTEM Charles Russell Luigs, Longview, Tex., assiguor to U.S. Industries, Inc., Longview, Tex. Filed Oct. 30, 1963, Ser. No. 320,109 5 Claims. (Cl. 103232) This invention relates to gas lift systems generally, and more particularly, to gas lift systems of the intermittent type wherein the gas is injected into the eductor tube at spaced intervals of time.

Gas lift systems are employed principally to lift well fluid out of oil or water wells when the energy of the producing formation is inadequate to produce the desired amount of fluid. In a gas lift system, gas under pressure is injected into the eductor tube at a point below the surface of the liquid in the tube. The gas exerts an upward force on the liquid above the point of injection causing it to travel through the eductor tube toward the surface.

To control the flow of gas into the eductor tube, a gas lift valve is usually located at the point of injection to open and close in response to predetermined conditions. The most commonly used gas lift valves are pressure operated, i.e., they open and close in response to the pressures of the ambient atmosphere. In intermittent gas lift systems, these valves are usually located so that they are predominantly controlled by the pressure of the lifting gas which can be controlled from the surface.

A plurality of such valves are spaced apart along the eductor tube. A plurality of valves are used because the point of injection of the gas will usually shift several times during the life of the well, as a result of the normal decline in pressure in the producing formation as the well is produced over a period of time, changes in the length of time between injections, etc.

Heretofore, these valves were usually spaced apart along the eductor tube according to a formula which included known values such as the opening pressure of the valve and the depth of the valve above, and also an unknown value known as the flowing gradient or spacing factor. This gradient was intended to be an equivalent gradient which would indicate the conditions in the eductor tube at the valve after each cycle. It was therefore an estimate of the pressure which would exist in the eductor tube at the valve due to the well fluid which did not travel completely out of the eductor tube but which fell back, plus the pressure exerted by the gas in the eductor tube. According to the formula, this gradient is multiplied by the depth of the valve above the valve being spaced to obtain the estimated back pressure at the valve above. This value is then converted to the equivalent feet of fluid and substracted from the distance the valves could be spaced apart if this back pressure did not exist.

Each valve spaced in accordance with this formula did not operate through the same range of pressure differentials while it was the lifting valve. This, in turn, caused the efficiency of the system to be different for each valve. Further, this heretofore used method of spacing intermittent valves, tended to locate the valves so that the point of injection of the lifting gas would shift to the next lower valve only when the pressure in the eductor tube at the lifting valve was equal to the well head back pressure plus the pressure exerted by the fluid fall back, and the gas column, as calculated by multiplying the estimated flowing gradient by the depth of the lifting valve. Thus, the amount of production per cycle would vary greatly during the period that each valve was the lifting valve.

Many different values have been assigned to this spacice ing factor. Experience indicates that it changes with changing conditions in the system such as depth, size of the eductor tube, volume of liquid being produced, etc. Experience also shows that whether or not it is estimated correctly will determine to a large extent the overall efficiency of the gas lift system.

It is an object of this invention to provide an intermittent gas lift system employing pressure operated gas lift valves which does not require an estimate of a flowing gradient to determine the spacing of the valves.

It is further an object of this invention to provide an intermittent gas lift system employing pressure operated flow valves in which the spacing of the valves is calculated from values which can be readily determined.

It is another object of this invention to provide an intermittent gas lift system having a plurality of pressure operated gas lift valves, wherein the pressure in the eductor tube opposite the valve which opens and injects suflicient gas to lift a slug of well fluid to the surface, i.e., the eductor tube pressure at the lifting valve, will be between a predetermined maximum and minimum percentage of the pressure of the lifting gas at the lifting valve to thereby improve the efficiency of the system, and also to keep the efliciency substantially uniform for each valve while it is the lifting valve.

It is another object of this invention to provide an intermittent gas lift system having a plurality of pressure operated gas lift valves wherein the valves are spaced to maintain a predetermined relationship between the pressures in the eductor tube opposite adjacent valves.

In an intermittent gas lift'system it is intended for the opening and closing of the valves to be determined principally by the pressure of the lifting gas. This allows the system to be controlled from the surface. The pressure in the eductor tube, of course, exerts a pressure on the valve element of the valve which also tends to open the valve. The amount of influence the eductor tube pressure has on the opening characteristics of the valve depends, of course, on the size of the pressure area against which it acts, which is in most cases the size of the opening through the valve.

Heretofore, it was common practice to use relatively small ported valves to keep the influence of the eductor tube pressure to a minimum so that the opening and closing of the valves could be more accurately controlled from the surface. Further, since the spread, i.e., the ratio of the effective area of valve element to the effective pressure area acted on by the lifting gas while the valve is closed, multiplied by the differential pressure across the valve when it opens, determines the pressure blowdown which will occur in the lifting gas pressure at the valve from the time the valve opens until it closes, it was heretofore considered desirable to use valves with small spreads since the differential across the valve, when spaced according to the formula discussed above, frequently became excessive, resulting in excessive blowdown of the injection gas pressure in the casing, and thereby creating inefficiency through consumption of excessive quantities of injection gas.

Therefore, it is another object of this invention to provide an intermittent gas lift system in which valves having a large "spread can be used since the maximum differential pressure across any lifting valve is a predetermined amount, which allows the maximum volume of lifting gas which will be consumed to be predetermined and the compensate to changing well conditions while maintaining substantially uniform operating efficiency.

It is another object of this invention to provide a gas lift system in which the liquid produced per cycle remains substantially constant from the same or adjacent lifting valves.

Other objects, features, and advantages of the invention will be apparent to those skilled in the art from a consideration of the specification, appended claims and attached drawings wherein:

In the drawings, FIG. 1 illustrates a well having a typical gas lift system installed in'it according to this invention and showing the condition of the well which is being unloaded; and

FIG. 2 is a similar illustration showing the well under producing conditions.

In accordance with this invention, the pressure operated va-lves, through which the lifting gas is injected into the eductor tube, are spaced to provide a predetermined maximum and minimum pressure differential across each valve at the time it is functioning as the lifting valve in the system. Thus, as opposed to the method of spacing the valves which was used heretofore, the spacing of the valves is independent of an estimated, indeterminant, gradient which was inherent in the heretofore commonly used spacing formula. Further, by spacing the valves in accordance with this invention, the pressure differential across the lifting valve can be maintained within the range which results in the most efficient operation of the system.

In the gas lift system of this invention, the pressure operated valves are spaced along the eductor tube according to the formula:

where S is the distance in feet between the valves; P is the pressure (in p.s.i.) of the lifting gas required to open the valve above the valve being spaced, when the pressure differential across the valve above the valve being spaced is: the maximum desired amount (this pressure will be hereafter called the design pressure); g is the fluid gradient (in p.s.i. per foot) of the fluid being lifted; y is the desired ratio of pressure in the eductor tube to the pressure of the lifting gas at the valve above when it is desired for the valve being spaced to become the lifting valve; and x is the desired maximum ratio of the pressure in the eductor tube, at the valve being spaced, to the design pressure, when the valve being spaced becomes the lifting valve.

The ratios represented by x and y in the equation can also be expressed as the desired maximum and minimum pressure differentials across the valves as a percentage of the design pressures. Thus, if it is desired for a valve to stop being the lifting valve when the pressure in the educator tube opposite this valve drops below 55% of its design pressure, and when at the same time the eductor tube pressure at the next lower valve is no more than 85% of the same design pressure, then x=.85 and 3 :55. However, when the pressure in the tubing equals 55% and 85%, respectively, of this design pressure, the differential pressures across the two valves as a percentage of the design pressure are 45% and 15%, respectively. Thus, whether x and y equal the ratio of the pressures .85 and .55 respectively or the percentage pressure differentials .15 and .45 respectively, the result is the same. Of course, when percentage pressure differentials are used, thd answer will be negative. However, this can be ignored.

The values used for x and y will, of course, vary with well conditions and with the anticipated function of the valve. Unloading valves, for example, need not operate efliciently, since they are used only briefly when thesystem is first placed in operation to remove any extraneous fluid in the well, after which they usually no longer function. For this reasonunloadingvalves can be spaced with relatively large differences in the maximum and minimum pressure differentials across the valves since the efliciency of their operation istnot important. For example, the maximum and minimum percentage differentials across the unloading valves may range from 10-70% of the respective design pressures.

Valves positioned below the static fluid level of the well which lift fluid coming from the producing formation are normally called operating valves. These valves should be spaced to operate at optimum efficiency. Maximum efficiency, of course, can be obtained at only one pressure differential across the lifting valve. It has been determined, however, that for average well conditions the operating valves in gas lift systems, when spaced according to this invention, will function at optimum efficiency when the eductor tube pressure opposite the operating valves range from approximately 5585% of their respective design pressures. Inserting these values into the spacing formula set out above results in the following preferred equation for spacing operating valves:

Valves spaced according to this equation then will function as lifting valves only when the eductor tube pressures opposite them range from 55-85% of their respective design pressures. In other words, when the eductor tube pressure opposite the lifting valve, in a system designed according to this invention, drops below 55% of the design pressure, the valve will cease to function as the lifting valve, this function being shifted to the valve below. Conversely, when the pressure opposite the lifting valve exceeds of the design pressure of the valve, the next valve above will become the lifting valve.

As explained above, the design pressure, P, in the spacing formula of this invention, is determined by the opening characteristics of the valve above and not by the opening characteristics of the valve being spaced. It is only in this way that the desired pressure relationship between the valves can be maintained for it is actually the opening and closing characteristics of the valve above which determines the conditions at the valve being spaced when it initially becomes the lifting valve. Once the valve being spaced becomes the lifting valve it will, of course, operate independently of the valve above and it will, in turn, determine the conditions which will exist at the valve below it when the time comes for the valve below it to begin functioning as the lifting valve.

Usually, it is desirable under conditions of maximum desirable differential, to have the surface opening pressure for each valve constant and only slightly less than the maximum available surface injection pressure so that all of the available energy in the available lifting gas can be utilized. This normally results in an increase in P for each valve spaced down the eductor tube to account for the increase in pressure due to the weight of the column of gas, when the pressure sensitive areas of the valves in the system are uniform. Thus the operating valves, in the preferred embodiment, are spaced increasing distances apart down the eductor tube.

However, if desired, the design pressure P of the valves can be made equal or progressively decreased as the valves are located down the eductor tube, which will cause the valves to be spaced the same distance apart or progressively closer together.

When using a constant surface opening pressure corresponding to specified opening pressures at the valves for a given pressure differential, some means should be provided to make certain that the valves above the desired point of injection do not also open when the lifting valve opens. Preferably, the desired control is obtained by using valves which are sensitive to eductor tube pressure so that the lifting gas pressure required to open the valves varies inversely with eductor tube pressure at the valves. This causes a valve located where the eductor tube pressure is within the preferred range to open at a lower lifting gas pressure than a valve located where eductor tube pressure is below the preferred range. Of course, when the eductor tube pressure at the valve is at the upper end of the preferred range, the lifting gas pressure required to open the valve will be slightly less than the design pressure. This variation in the lifting gas pressure required to open the valve is usually not significant.

To more clearly explain the invention, a typical gas lift system having valves spaced according to the preferred embodiment of the invention will be described in detail below in connection with FIGS. 1 and 2 of the attached drawings which are schematic drawings illustrating different well conditions which could exist in a typical gas lift system ararnged according to this invention.

Illustrated schematically in the figures is well casing which extends from the surface 11 of the ground to a fluid producing formation 12. Extending into the well casing to a point adjacent the producing formation is well tubing 13 which, in this case, is also the eductor tube. The tubing combines with the well casing to provide casing annulus 14 which is sealed off from producing formation 12 by means of packer 15. At the surface, the casing annulus is connected to a source of gas through line 16. Any of several well known means can be used to control the flow of gas through line 16 into the annulus. In the system illustrated a choke 17 is used. The well tubing 13 extends out of the well at the surface and is connected to the usual separator and storage tanks, etc. (not shown).

Attached to well tubing 13 are a plurality of pressure operated gas lift valves which are generally designated by the letters A, B, C, D, E, and F. These valves are spaced along the tubing the distances S with the spacing of each individual valve indicated by the suffix of the letter of the particular valve.

Assume that valves A, B, and C are unloading valves and are spaced according to the equation:

g '9 whereas valves D, E, and F are operating valves and are spaced according to the equation:

Valve C can function as either an unload-ing valve or an operating valve depending on how high the well fluid accumulates in well tubing 13. For example, if, in the embodiment illustrated, the working fluid level of the well, i.e., the height in eductor tube 13 to which fluid will rise between cycles, is above valve C sufficiently for the pressure opposite the valve when the well has been unloaded, to be between 55'85% of the design pressure of the valve, then the valve will initially function as an operating valve. However, if the working fluid level is such that the pressure opposite the valve is less than 55%, the desired minimum percentage of the design pressure of the valve, then it will operate only as an unloading valve and valve D will be the first operating valve in the system. For this reason, if it is not known, an estimate of the height of the static fluid level is usually made prior to the running of the valves so that a depth will be established at which the spacing should change from unloading to operating.

In the example illustrated, however, assume that the working fluid level 18 is at a point such that the pressure opposite valve C is between 55% and 85% of the design pressure of the valve. Valve C will then be the top operating valve.

Usually, the well is filled with liquid while the tubing with the attached gas lift valves and packer are being installed. The first operation is to unload the well 6 by removing the excess liquid from the casing annulus and tubing in the manner well known in the art. In the drawings the level of the fluid in the annulus is indicated by the number 19.

As explained above, unloading valves are usually spaced relatively far apart since high efficiency is not required during the unloading process. Of course, they should be spaced sufliciently close together to perform their function within a reasonable length of time and without squandering gas. The equation used to space the unloading valves in the embodiment illustrated has been used with consistently good results.

In the unloading operation, gas can be injected continuously or intermittently as desired. However, when the well is unloaded, some means must be provided for controlling the flow of lifting gas so that the operating valves can be opened and closed at desired intervals.

One common method of doing this is to limit the rate at which gas enters the casing annulus by restricting the opening in the supply line with a properly sized choke, so that the rate of gas pressure build-up in the casing annulus, produces the desired interval of the time between injections, and also the desired length of injection. With this type of control the capacity of the casing annulus or gas carrying conduit in the well should be such that the volume of gas obtained when the pressure therein is blown down to the closing pressure of the valve, plus the volume of gas which enters while the valve is open and blowdown is occurring, is sufficient to lift a slug of well fluid to the surface. Otherwise, means needs to be provided to by-pass the choke to supply the necessary volume of gas while the valve is open.

When valves are spaced according to this invention, the maximum blowdown which will occur can be calculated by multiplying the valve spread by the predictable maximum differential which can exist across the valves. In this manner, the valve spread and choke size can be selected to insure that the desired volume of gas is available while the lifting valve is open.

Instead of a choke, a time cycle controller can be used to periodically open a valve in the gas supply line and inject gas into the casing long enough to open the operating valve or valves and lift the fluid slug to the surface. Also, many installations employ both the choke and time cycle controller in combination. Other well known arrangements could also be used.

Assume, for purposes of explanation, that the lifting gas is being supplied to annulus 14 through a choke, that the well is unloaded and conditions are as shown in FIG. 1, and that valve C is the lifting valve with the pressure in the tubing at valve C, P of the design pressure of valve C. Under these conditions, the pressure at valve D, P is equal to .85 P +S g, which is normally greater than the opening pressures of valves C and D. Thus, when the pressure of the lifting gas at valve C is sufficient to open the valve, all of the gas will flow through this valve into the tubing. (P and P are the design pressures of valves C and D, respectively.)

Well conditions immediately after valve C closes after having injected sufficient gas into the eductor tube to lift a slug of well fluid out of the tubing, are shown in FIG. 2. The pressure in the annulus, P equals the closing pressure of the operating valve, valve C. The pressure in the tubing at valve C, P will now equal the pressure exerted by the fluid which fell back when valve C is closed, plus pressure of the gas above the valve, and the well head back pressure, a value indicated by the letter X. The pressure in the tubing opposite valve D, P will now equal S g+P Well fluid is continuously entering the tubing from the format-ion so shortly after valve C closes, the pressure in the tubing opposite valves C and D will begin to increase. Further, since gas is being continuously injected into the annulus through choke 17, the pressure is continuously rising therein also. The valves are responsive to both pressures. Thus, Whether or not valve D opens before valve C will depend upon the relative rates these pressures increase.

For example, the rate of build-up of pressure in the casing annulus may be such that the well fluid returns to the level shown in FIG. 1 before there is sufficient gas pressure in the casing annulus to open either valves C or D. Usually, however, the operator of the system will desire to increase the drawdown across the formation by lowering the pressure exerted thereon by the fluid in the tubing, so he will use a choke which will inject :gas at a rate suflicient to open valve D before valve C is opened. If the pressure differential across valve D is low, which will usually be the case, if valve C is still the lifting valve, the rate of flow of gas through valve D will not be sufficient to keep the pressure in the casing from continuing to increase. Also, since the pressure in the tubing usually increases faster than the pressure in the annulus, the differential across valve D will continuously decrease, further reducing the rate of gas flows through it. Thus the pressure in both the tubing and casing will continue to rise until valve C opens, injecting gas to lift the fluid above it to the surface. When valve C opens, the pressure of the injected gas under the fluid slug plus the fluid head between the valves stops the flow of gas through valve D.

Actually, the gas which passes through valve D is not wasted. It aerates the fluid column above the valve which extends its length and increases the volume of fluid above valve C. This results in an increased volume of fluid being produced each cycle. Also, by aerating the portion between valve D and valve C, the gradient of this portion of the fluid will be reduced so that after valve C opens and lifts the fluid above it to the surface, the eductor tube pressure atvalve D will be less than it would be otherwise. This increases the bottom hole pressure drawdown which usually causes an increase in the rate of flow of well fluid from the formation. Even if the rate of fill-in remains constant, the resulting decrease in pressure opposite valve D will cause it to open each time with a larger differential across it than before, causing a greater flow of gas through the valve, which results in an even smaller amount of fluid being left above it after each cycle.

This process will continue until the rate of gas flow through valve D, when it opens, equals the rate of flow of gas into the annulus through the choke. When this occurs, casing pressure can no longer. increase, and only fluid filling in from the formation can open valve C. These conditions, in turn, will last until the fluid load remaining above valve D is such that it will remain closed until the differential across it-is suflicient to lift the fluid above it to the surface. In the embodiment illustrated, which is the preferred form of the invention, this differential would be 15 of the design pressure of valve C. At this point, valve D will become the lifting valve.

Thus, valves spaced according to this invention will begin to function as lifting valves when the differential pressure across them is a predetermined value and stop functioning as lifting valves when the differential pressure across them drops to another predetermined value. Further, it should be noted that these differential pressures are based on the design pressure of the valve which is to stop functioning as the lifting valve. For example, in the example described above, valve D became the lifting valve when the differential across it was approximately 15% of the design pressure of valve C, not valve D.

The process described in connection with valvesC and D will continue with each lower valve functioning in turn as the lifting valve until equilibrium is obtained or valve F becomes the lifting valve. Whether equilibrium occurs will depend on the relative rates of liquid withdrawal and fluid fill-in. If the fluid withdrawal rate is greater than the fluid fill-in rate, then the point of injection will move down from valve to valve until it reaches the bottom valve. However, a point may be reached above the bottom valve where the fluid fill-in rate and the rate of 8 fluid withdrawal are balanced and the .point of injection, i.e., the lifting valve, will remain the same.

It has been determined that when the opening pressure of the operating valves are close to the same and they are spaced according to this invention so that the differential across the lifting valve is in the preferred range, the production per cycle from this valve and either of its adjacent operating valves, if any, will be approximately constant. This is a particularly valuable attribute of the system where the production of the well is predetermined by a regulatory body and must be accurately controlled, for it allows the operator to control his daily production by controlling the number of injection cycles per day. Thus by an appropriate adjustment of his surface'controls he can predetermine, with reasonable accuracy, the production rate of the well.

From the foregoing, it will be seen that this invention is one well adapted to attain all of the ends and objects hereinabove set forth, together with other advantages which are obvious and which are inherent to the method and apparatus.

It will be understood that certain features and subcornbinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims.

As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be undersood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

The invention having been described, what is claimed 1. An intermittent .gas lift system for periodically producing fluid from a well bore comprising in combination an eductor tube extending into the well fluid; means for providing lifting gas adjacent the eductor tube; a plural ity of pressure operated gas lift operating valves mounted on the eductor tube and controlled by both the pressure of the lifting gas and the pressure in the eductor tube, the valves being located below the static fluid level of the well, said valves, except the uppermost valve, being spaced so that the minimum and maximum pressure in the eductor tube opposite the valve which opens and injects gas into the eductor tube to lift the fluid above it to the surface always lies within the range of 55 to percent of a predetermined value, said value being the gas pressure required to open the next valve above when the pressure of the well fluid adjacent the said next valve above is at said minimum pressure.

2. A .gas lift system for lifting fluid from a well, said well having a flow tube extending into the well and a well casing surrounding the flow tube to provide an an nular space therebetween; means for introducing lifting gas under pressure into the annular space; and a plurality of pressure operated operating valves carried by the flow tube and located below the static fluid level of the well, said valves being arranged to control the flow of lifting gas from the annular space into the flow tube in response to the pressure in the flow tube and the annular space, the v valves, except the uppermost valve, being spaced from each other such that when the well fluid in the eductor tube is exerting a pressure at one valve equal to 55% of the gaspressure required to open the valve, it will also be exerting a pressure equal to 85% of the gas pressure required to open said one valve at a point adjacent the next lower valve.

3. A gas lift system for lifting fluid from a well comprising an eductor tube in the well through which the well fluid is lifted by means of lifting gas injected below the surface of the fluid; a plurality of pressure operated operating valves connected to the eductor tube at spaced pointsto control the flow of gas into the eductor tube;

and means for supplying lifting gas to the valves, each valve being responsive to the pressure of the gas and th pressure in the eductor tube and, except for the uppercrnost valve, each valve being spaced from the next valve above a distance determined by the equation:

where S is the distance in feet the valve should be placed 'below the next valve above, x is the maximum desired ratio of the pressure of the Well fluid in the eductor tube at the valve to the pressure of the lifting gas at the valve being spaced when the valve opens to inject lifting gas into the eductor tube in sufiicient quantities to lift a portion of the well fluid above it from the eductor tube, y is the minimum desired ratio of the aforesid pressures; P is the gas pressure necessary to open the valve above the valve being spaced, in p.s.i., when the ratio of pressures across the valve is the desired minimum; and g is the gradient of the fluid being lifted in p.s.i./ ft.

4. The gas lift system according to claim 3 in which x:.85 and y=.55.

5. The gas lift system of claim 3 in which P is such that the same pressure is required at the surface to open each valve when the ratio of the eductor tube pressure at the valve is 55% of the lifting gas pressure at the valve.

References Cited by the Examiner UNITED STATES PATENTS 10 2,292,768 8/1942 Parker 103233 2,673,568 3/1954 Bufrington 103233 2,698,024 12/1954 Canalizo 103-233 DONLEY I. STOCKING, Primary Examiner.

15 LAURENCE v. EFNER, MARK NEWMAN,

Examiners.

G. M. THOMAS, W. I. KRAUSS, Assistant Examiners. 

1. AN INTERMITTENT GAS LIFT SYSTEM FOR PERIODICALLY PRODUCING FLUID FROM A WELL BORE COMPRISING IN COMBINATION AN EDUCTOR TUBE EXTENDING INTO THE WELL FLUID; MEANS FOR PROVIDING LIFTING GAS ADJACENT THE EDUCTOR TUBE; A PLURALITY OF PRESSURE OPERATED GAS LIFT OPERATING VALVES MOUNTED ON THE EDUCTOR TUBE AND CONTROLLED BY BOTH THE PRESSURE OF THE LIFTING GAS AND THE PRESSURE IN THE EDUCTOR TUBE, THE VALVES BEING LOCATED BELOW THE STATIC FLUID LEVEL OF THE WELL, SAID VALVES, EXCEPT THE UPPERMOST VALVE, BEING SPACED SO THAT THE MINIMUM AND MAXIMUM PRESSURE IN THE EDUCTOR TUBE OPPOSITE THE VALVE WHICH OPENS AND INJECTS GAS INTO THE EDUCTOR TUBE TO LIFT THE FLUID ABOVE IT TO THE SURFACE ALWAYS LIES WITHIN THE RANGE OF 55 TO 85 PERCENT OF A PREDETERMINED VALUE, SAID VALUE BEING THE GAS PRESSURE REQUIRED TO OPEN THE NEXT VALVE ABOVE WHEN THE PRESSURE OF THE WELL FLUID ADJACENT THE SAID NEXT VALVE ABOVE IS AT SAID MINIMUM PRESSURE. 